This invention relates generally to electroacoustic transducers and more particularly to electroacoustic transducers having an improved watertight seal for increasing operating efficiency and manufacturing ease while decreasing overall transducer size.
As is known in the art, electroacoustic transducers are used in underwater environments to convert electrical energy into acoustic energy and likewise, acoustic energy into electrical energy. When acoustic energy is propagated, the device is generally referred to as a projector; whereas, when such energy is received, the device is referred to as a hydrophone. One hydrophone application is a sonobuoy which often contains a plurality of acoustic transducers. The sonobuoy may be discharged from an aircraft and upon impact, the transducers are ejected and hang several hundred feet down into the water from a buoy which remains on the surface and which contains electrical transmission apparatus. The transducers receive acoustic energy or signals and convert such signals into electrical signals. Such electrical signals are transmitted to the buoy by an interconnecting cable and receiving apparatus, for example disposed on an aircraft or boat, receives such electrical signals. With this arrangement, activity in the water, such as the passing of a ship, can be detected.
Some electroacoustic transducers include a resilient shell which moves or vibrates in response to excitation by either an electromechanical driving mechanism or acoustic energy, in order to propagate or receive acoustic energy, respectively. Several types of resilient shells are conventionally used, such as an elliptical shaped shell having open end portions or a cylindrical shaped shell having one or more slots disposed parallel to the axis of the cylinder. The former type of shell provides what is generally referred to as a flextensional transducer and the latter shell provides a split-ring or split-cylinder transducer. When a split-cylinder transducer has more than one slot, it may be referred to as a multi-slotted cylinder transducer.
Conventional acoustic transducers operating as hydrophones are driven by a variety of electromechanical mechanisms which include natural piezoelectric (e.g. quartz), synthetic piezoelectric (e.g. a ceramic), magnetostriction, variable reluctance (e.g. a magnetic drive), and moving coil drivers. In flextensional transducers and multi-slotted cylinder transducers, the driver is often disposed in a columnar arrangement between opposite ends of the shell. For example, in the case of a flextensional transducer having an elliptical shaped shell, the driver may be disposed between the ends of the shell along the major axis of the ellipse. With this arrangement, when the driver is positively energized, it pushes outward on the ends of the elliptical shell along the major axis and the sides of the shell along the minor axis of the ellipse move inward. When the driver is negatively energized (i.e. when the input signal corresponds to the negative half cycle of the sine wave energizing signal), the ends of the elliptical shell along the major axis move inward and the sides of such shell along the minor axis thereof move outward. In this way, acoustic energy is propagated by periodic excitation of the driver. In split-cylinder transducers, the driver is commonly provided in a cylindrical shape and is coupled to the interior of the cylindrical shell. When such driver is positively energized, the slot is forced open or widened, thereby causing the cylindrical walls to move in the water environment. When the driver is negatively energized, the resilient cylindrical shell contracts to its initial shape. In this manner, acoustic energy is propagated by the periodic excitation of the driver.
The interior of conventional acoustic transducers may be either fluid filled or gas filled. In either case, it is necessary to seal the interior of the shell from the surrounding water environment. One way known in the art for providing a watertight seal is to cover the open ends of the transducer shell with metal end caps or plates spaced from the shell and to cover the entire assembly (including the slot of the split-cylinder transducer) with a flexible cover or "boot." With this arrangement, the shell is free to move upon excitation by the driver mechanism or acoustic energy. However, the movement of the shell may be somewhat inhibited or restricted by the coupling of the shell to the non-flexible metal end caps via the boot. That is, while the flexible boot will move somewhat in response to shell movement, the movement of the boot is restricted by the end caps disposed thereunder. Moreover, inhibition of the shell movement adversely affects the transducer efficiency (i.e. the ratio of acoustic energy output to electrical energy input in the case of a projector and the ratio of electrical energy output to acoustic energy input in the case of a hydrophone) since energy is used in stretching and shearing the boot instead of in propagating acoustic energy.
One way known in the art to improve the efficiency of electroacoustic transducers utilizing conventional watertight seals or boots is to provide slack in the boot material (i.e. a "loop" of boot material between the ends of the shell and the metal end caps spaced therefrom, as described in U.S. Pat. No. 4,949,319 entitled "Sonar Transducer Joint Seal" with inventors Richard W. Boeglin and Arthur B. Joyal, issued on Aug. 14, 1990 and assigned to the assignee of the subject invention. With this arrangement, when the shell moves, the boot is free to move to a greater extent before being restricted by the metal end caps. In fact, this loop feature has also been applied to the slot of split-cylinder transducers, as described in U.S. Pat. No. 5,103,130 entitled "Sound Reinforcing Seal for Slotted Acoustic Transducer" with inventors Kenneth D. Rolt and Peter F. Flanagan, issued on Apr. 7, 1992 and assigned to the assignee of the subject invention. However, while these loop arrangements improve transducer efficiency by decreasing restraint on the shell's motion, further efficiency improvement may be desirable.